The invention is directed to an actuating drive, particularly for control valves and shut-off valves or the like, having a safety device that automatically brings an actuation element into a safe position, particularly a closed position, at a limit temperature that exceeds the normal temperature range, particularly under fire conditions.
"Fire proof" fittings are being increasingly demanded in automation technology. An actuating element is deemed "fire proof", for example according to the British standard BS 6755, Part II, 1987, when it is automatically moved into a safety position when a fire erupts and can be held therein over a specific time, for example 30 minutes. Given combustible materials, the safety position is thereby usually the closed position. Typically then for a "fire proof" arrangement, an actuating drive has a safety device that, when a fire breaks out, charges an actuation element in such a way that the closed position is achieved.
Energy stores in the form of springs or, given pneumatic drives, appropriate compressed air supplies or reservoirs have hitherto been employed as safety devices of this type. Since certain temperatures above the limit temperature of the energy store components are reached or even exceeded, the force of the energy store which holds the actuation element in the safety position for typical drive embodiments is lost because, for example, the membrane of a membrane drive melts or, respectively, the springs loose the hardness and become "soft". Ball valves and slides can remain in their closed position, since the medium presses the closing element against the outflow opening as a consequence of the static pressure difference. This is the reason that only ball valves and slides were previously recognized as being "fire proof".
In lift positioning devices, the valve cone is pressed up given outage of the force from the energy store if no additional measures are undertaken. As known, such measures can be making optimally long the time span until the loss of the safety position, for example on the basis of water showers, encapsulating the device in a fire proof housing, employing thermally insulating swelling coats (agents that form insulating layers) or durably obtaining the safety position on the basis of mechanical latch devices (interlocking) or on the basis of refractory spring materials (for example, "Inconell").
These measures are either extremely complicated, maintenance-unfriendly, costly or have a limited or, respectively, unreliable effectiveness.